Power steering systems have been known for a long time, in which the torque the driver applies to a steering wheel is amplified and transmitted to the steered wheels, generally the front wheels of the vehicle, by means of an actor (e.g., a hydraulic cylinder, an electric motor or the like). Although these systems have greatly reduced the steering effort for the driver, they were still found not quite satisfactory. When running at high speed (e.g., on a highway), the driver usually applies only small turns to the steering wheel and tends to be uneasy if he hardly feels counter-torque from the steering wheel. This imposes an upper limit for torque amplification in the power steering system which causes a substantial counter-torque to be felt when maneuvering at low speed, where large steering angles are required frequently. More recently, adaptive power steering systems have been introduced in which this problem is overcome by using different torque amplifications at high and low speeds.
A new problem has been created by these adaptive power steering systems which can be extremely irritating for the driver: If the vehicle is braked abruptly while cornering, and its speed drops below the switchover threshold between different torque amplification ratios, the counter-torque experienced by the driver decreases abruptly, giving him the feeling that the vehicle has lost hold on the ground. Since abrupt braking will in most cases occur in critical situations where the driver's attention is absorbed with maneuvers necessary to avoid an accident, there is a risk that he will not react adequately upon the change of counter-torque.
AFS (active front steering) is a technique which allows variation in the transmission ratio of the steering system depending on vehicle speed. An aim of AFS is similar to the adaptive power steering systems mentioned above: to reduce the driver's steering effort in small-radius cornering maneuvers at low speed, while maintaining a well-felt counter-torque at high speed. Here, the problems caused by a switchover between different transmission ratios can be more serious still than with adaptive power steering, since the front wheel road angle associated to a given turning angle of the steering wheel depends on the transmission ratio.
In view of the foregoing, at least one object of the present invention is to provide an adaptive steering control system which will avoid any control operations which might risk taking the driver by surprise. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.